1. Field of the Invention
The present invention relates to an improved process for the thermomechanical treatment (TMT) of copper alloys and more particularly to a TMT of copper alloys to increase the strength of the alloy and maintain high electrical conductivity.
2. Description of the Prior Art
Copper has long been known for its excellent electrical conductivity properties. The requirement for excellent conductivity in environments of moderate elevated temperature and mechanical stress has required the utilization of copper base alloys which are formed by the addition of alloying elements such as, for example, beryllium, nickel, cobalt and zirconium. Typically, alloy strengthening procedures start with a solution heat treatment which reduces the electrical conductivity prior to the precipitation hardening which is aimed to improving the material strength characteristics. Electrical properties may be improved somewhat by precipitation hardening of the alloy by removing the alloying elements' atoms from the copper over-saturated matrix.
The prior art alloying process may be explained in reference to a commercially available Cu-Be-Ni alloy UNS C17510. In accordance with ASTM Standard B534-82, UNS C17510 has the following composition:
TABLE A ______________________________________ Element Wt % ______________________________________ Beryllium 0.2-0.6 Cobalt 0.3 max Nickel 1.4-2.2 Nickel & Beryllium & Copper 99.5 (min) Iron 0.10 (max) ______________________________________
The material in the solution hardened and aged condition exhibits an ultimate tensile strength of on the order of 110-140 ksi (kilopounds per square inch), a 0.2% yield strength of on the order of 100-120 ksi, a tensile elongation of about on the order of 5-20%, a hardness (R.sub.B) of about on the order of 95-102 and an electrical conductivity (%IACS) of about on the order of 58%.
Typically, the alloy is first solutionized (annealed) at a temperature of about 900.degree. F. (1650.degree. C.) and is then quenched in water at room temperature. This treatment puts the material in the A (known as TBOO according to ASTM 601 standard) temper. In the process of solutionizing, a solid state solution is formed such that the Ni, Be and other elements are uniformly distributed by atom diffusion in the copper matrix. The quenching "freezes" the atoms in their distributed state. This atomic alignment is, however, unstable since it is oversaturated. Moreover, the material is soft, and therefore, it is cold worked, such as by cold rolling. For example, in the common full hard condition the material is cold worked to reduce its thickness by 37% to put it in what is known as the H (TDO4) condition. Then the alloy can be precipitation hardened (aged) to put it in the HT (THO4) condition. The most common aging treatment is 3 hours at 482.degree. C. (900.degree. F.), but may also be from 2-72 hours at temperatures 300.degree.-500.degree. C. (570.degree.-930.degree. F.).
The desired hardening of the alloy as a result of this treatment results from precipitation hardening of the NiBe.sub.2 intermetallic phase compounds (IC) formed out of the supersaturated solid solution of Be and Ni in copper that was freezed by the water quench from the solutionizing temperature. The resulting IC precipitates are very hard, very small and strong. This improvement in strength is achieved without significant brittleness. The alloy is usually solutionized at 900.degree. C. (1650.degree. F.) when supplied by commercial vendors, and may be referred to as 1650 HT. However, it can also be solutionized at higher temperatures such as 955.degree. C. (1750.degree. F.) to give a higher strength with an accompanying small drop in ductility and electrical conductivity. The higher strength results from the higher temperature which will put more IC atoms in solid solution. This higher solutionizing temperature variation of UNS C17510 will hereinafter be referred to as 1750 HT. In both cases the alloy is also specified by the composition Cu-0.4 Be-2Ni.
Several prior art references relate to the treatment of copper alloys. For instance, U.S. Pat. No. 4,179,314 to Wikle relates to the thermal treatment of Be-Cu alloys to optimize conductance and mechanical properties at elevated temperatures. The treatment taught by Wikle comprises the sequence of annealing followed by quenching, then cold working and a second optional annealing followed again by quenching and cold working. Next is an initial age hardening treatment, a secondary age hardening treatment, if necessary, straightening the alloy, and then stress relieving the straightened alloy. The treatment of Wikle is used to provide shaped beryllium-copper alloys useful in fabricating rotor wedges for electrical generators which retain optimum notched stress rupture resistance and thermoelectrical conductivity at high operating speeds and temperatures.
U.S. Pat. No. 3,573,110 to Ence discloses a process for obtaining high conductivity copper based alloys in which the alloy is heated at 700.degree.-1000.degree. C. for at least one half hour, hot rolled, cooled to below 300.degree. C. at a rate of greater than 550.degree. C./hour, cold rolled below 200.degree. C. and then aged at 250.degree.-575.degree. C. for at least one hour.
Nippert et al, in U.S. Pat. No. 2,879,191 disclose a method of producing heat treated copper zirconium alloys for notched articles such as electrical conductors, commutators or the like that are stronger in the traverse-to-cold-working direction than in the parallel-to-cold-working direction so that the articles are not weakened by the presence of the notches.
Other examples of TMT of copper alloys are taught in:
U.S. Pat. No. 3,882,712 to Shapiro et al., PA1 U.S. Pat. No. 3,717,511 to Wallbaum, PA1 U.S. Pat. No. 3,046,166 to Hartmann.
Rosen and Atzmon, in an article entitled Thermo-Mechanical Treatments of 18 Ni (300) Maraging Steel, discuss an investigation on the effect of thermo-mechanical treatments on the tensile properties of steel under various conditions such as solution treatment, aging and overaging. Experiments are reported involving aging, rolling and reaging to determine the effects of various parameters on hardness, tensile properties and the amount of reverted austenile. Consideration is not given, however, to the TMT of copper alloy to enhance its strength without degrading its conductivity properties.
The prior art teachings are not directed toward the fabrication of a high strength, high conductivity copper alloy suitable for use in a high field fusion reactor such as disclosed in U.S. Pat. No. 4,363,775 entitled Controlled Thermonuclear Fusion Device and Method, incorporated herein by reference. For such use, the copper alloy of the toroidal field coils are exposed to large operating stresses and must carry high current densities. It is desirable to fabricate a suitable high strength, high conductivity copper alloy from commercially available starting materials. Such an alloy may be a Cu-Be-Ni alloy from commercially available UNS C17510 starting materials such as 1650 HT or 1750 HT. The prior art fails to teach how such improved strength and high conductivity copper alloys can be made.